Sensor elements of this kind are used, in particular, in the motor vehicle field to measure air-fuel gas mixture compositions. In this case, these sensor elements are also designated as “lambda probes”, and they play an important part in the reduction of pollutants in exhaust gases, both in Otto engines and in Diesel technology.
In the meantime, these types of sensor elements are understood to come in a variety of numerous specific embodiments. The exemplary embodiments and/or exemplary methods of the present invention is basically applicable to most of these known specific embodiments. A particular emphasis of the exemplary embodiments and/or exemplary methods of the present invention, to which the invention is not limited, however, is supposed to apply to so-called proportional probes or broadband probes, in which a gas mixture composition is determined, for example, via a current flowing through the solid electrolyte.
In many of such sensor elements, the calibration of the sensor elements after manufacturing plays an important part. This calibration is used, for instance, to adjust for production tolerances, and is performed for the purpose of setting rigidly specified electrical properties of the sensor elements.
One exemplary embodiment of such a calibratable sensor element which is able to be used within the scope of the exemplary embodiments and/or exemplary methods of the present invention, or modified according to the present invention, is discussed in DE 103 45 141 A1. This document refers to a sensor element in which a first electrode, lying outside, and a second electrode, lying inside, are separated by a solid electrolyte foil. The inner electrode is able to have gas from a measuring gas chamber applied to it, via a gas access hole. In order to reach the inner electrode, the gas has to penetrate a diffusion barrier in the process, via which a limiting current of the sensor element may be adjusted.
The diffusion barrier is at least locally closed by an at least extensively gas-tight cover layer. This cover layer, which may be produced, for instance, by printing a paste of zirconium oxide on the diffusion barrier, followed by a sintering step, may subsequently be locally cut open by specific laser cuts, so that a shortening of the diffusion path can be effected. The limiting current of the sensor element may be specifically adjusted by this shortening of the diffusion path. A base limiting current is produced in this instance by a frontally open input region of the diffusion barrier, which is able to be increased in a targeted manner by laser cuts.
The main challenge in such calibratable sensor elements, as in the sensor element of DE 103 45 141 A1, in which the calibration takes place by the local removal of a cover layer, lies in producing a cover layer that is free of cracks and is stable for a long time, in a calibration region above the diffusion barrier that is open to calibration. However, such a cover layer, that is crack-free and stable for a long time, assumes a good lamination between the diffusion barrier and the cover layer. This may usually be ensured by using a matched material/paste combination for the functional layers, as well as a controlled production process. Such lamination superstructures (layers) normally receive an additional pressure in a laminating process which, prior to a sintering step, further stabilizes the entire composite.
In the case of newer, linear diffusion barrier-electrode designs, that is, superstructures in which the diffusion barrier and the inner electrode are situated in a common inner layer, as in DE 103 45 141 A1, for instance, and in which an open gas access hole, which also functions as an adjustment hole, is present, this composite, when being laminated, is reinforced only partially by the laminating pressure. The electrode and the diffusion barrier are also situated in one plane in many radial designs, so that a similar problem arises. Whereas the cover layer-diffusion barrier region, lying below the solid electrolyte layer used as pump foil, on the contact terminal side, is exposed to a high laminating pressure, this is not so in the region of the gas access hole or the adjustment hole. Because of the nonhomogeneous laminating pressure on the sensor element in the vicinity of the bore hole edges, additional shear forces act which are able to destabilize this region even more, and are able to lead to stresses and cracks in the cover layer. Such stresses and cracks may, however, give rise to a lot of scrap and a great variation in the limiting current (from here on also designated as Ip) and in the k value.
The k value, in this context, is a measure for the pressure dependence of the limiting current, and is discussed in WO2007/104621, for example. The high variation of the limiting currents is compensated for again, as a rule, by the calibration, the laser calibration, for example. However, in this manner one is not able to compensate for the k value variation, as a rule. On the contrary, it may be established that the calibration also influences the k value, since during the calibration, the ratio between the gas phase diffusion and the Knudsen diffusion is changed. Because of this, the variation in the limiting currents acts on the variation in the k values in addition, via the calibration. In the case of a great variation in the limiting currents, the adjustments have to be of different sizes. The k value, in this instance, is also varied correspondingly differently because of the calibration.
The variation, particularly the variation of the k values, is further influenced by cavities which ensure the gas access to the diffusion barrier. These cavities are formed, for example, by the lower edge of the gas access hole and the solid electrolyte foil. This gas access hole to the diffusion barrier, that is able to be produced only with difficulty, makes an additional contribution to the variation in the limiting currents and the k values. This has a subordinate meaning for the limiting currents, since a limiting current may be adjusted subsequently. Different diffusion resistances in the input region, however, influence the k value decisively. If the proportion of the diffusion resistance over the diffusion barrier remains essentially stable, and if the proportion of the gas phase diffusion resistance of the overall diffusion resistance is changed by a narrowing in the cavity of the input region, the k value changes considerably. Because of the additional adjustment of the limiting currents, this influence will be reinforced under certain circumstances. In the usual designs, the k value may fluctuate in a range between 0.95 and 1.25 bar.